With the recent expansion in the use of computers and computer terminals it is not unusual to have a large number of computers and terminals within a limited local area. It is very desirable to couple these units together to allow sharing of resources and provide the ability to permit a single terminal to access a plurality of other terminals and computers. The medium alowing these features is a local area network system which provides switching and data distribution for a communication medium specifically tailored or local area transmission distances and which allows computers to communicate with each other and further allows any single terminal to have access to a multiplicity of computers and peripheral equipment.
An important aspect of any local area data distribution system is its network topology or interconnection scheme between data processing devices and network nodes. These topology aspects are important since the particular topology selected impacts the administration, complexity and overall cost of constructing and operating the system. A local area data distribution network may conceivably assume many interconnection topologies which may range from complex arbitrary topologies to more basic structured topologies. The basic structured topologies are normally constrained to certain basic types in order to limit the control complexity of the network nodes with the most common basic types being the star, the ring, and the bus transmission system topologies.
In a star network topology each data processing device is connected to a common central switching node which is operative for interconnecting the data processing stations to each other or to a gateway coupling it to a wide area network comprising a plurality of local data distribution networks and computers. The predominant wiring pattern is from point to point, that is, from a plurality of individual data processing devices to a central host switching node. The star network topology has the advantage of providing superior privacy since each subscriber line is dedicated, and good security, since the control switch node may be securely housed and is centrally controlled, the network is centrally maintained and it also provides high reliability.
The ring and bus network topologies are distributed type topologies and eliminate the central switching node that is the heart of the star network topology. Hence, these topologies include some of the control functions within each of the data processing devices connected thereto. Network maintenance is normally more difficult than in the star topology because of the distributed nature of the network.
In the ring network topology, transmission is from node to node around a closed loop and each node may alter the dat passing therethrough. Each data processing device is connected to a separate node and intercepts only data messages specifically directed to it. Since the data flows through each node and since the nodes are distributed, no central node may be separately secured and hence, the security and privacy of a ring network topology is somewhat less than that of the star network topology. Since all nodes are in one closed loop, failure of a single node may render the whole ring network inoperative. The closed loop topology also limits flexibility in some arrangements where adding new nodes or data processing stations may result in the loss of data.
Data transmission in a bus network topology is typically broadcast from one source to all other devices on the same bus but normally only accepted by the device to which it is specifically addressed. Individual data processing devices are programmed to recognize data messages addressed to or intended for them as they pass by on the bus. The reliability of the bus network topology in terms of network node failure is greater than that of the ring network topology although a break in the bus may be catastrophic. There is also greater flexibility in adding new data processing devices to the system than is normally possible with the star topology since no wiring reconfiguration is required.
Yet another data distribution topology, currently being used is the petal arrangement or star ring hybrid arrangement in which the ring data transmission bus assumes a star like configuration. Each individual data station is located on a separate petal in which the data transmission path starts from and returns to a relay switch located at a central control node while passing through a data station located in the petal. The relay switch is utilized to short and thereby disconnect any petal containing a faulty station from the ring network and thereby maintain the continuity at operation of the remaining ring transmission path. The control of data flow arrangement is essentially the same as that of a conventional ring topology.
In some modifications of the above arrangement an individual petal may include more than one data station, however the petal transmission loop still remains part of the ring and the data flow is unidirectional in the petal's outgoing and incoming transmission paths in agreement with the direction of data flow in the ring network. In both arrangements the performance achieved and privacy considerations are similar to those of the basic ring data distribution topology.
In still another data distribution topology, star configured wiring is utilized with a functional OR type logic gating bus arrangement in which a plurality of data stations are coupled to an OR-gating bus. The OR-gating bus is in turn coupled to a bridging hub which joins a number of such OR-gating busses into a bus structured network. In this arrangement each station seeking permission to transmit data looks for the presence of another transmitting station on the ORing bus arrangement before proceeding to transmit data. A major disadvantage of this arrangement is that signal propagation delays may allow several stations to assume data transmission permission with resulting data collisions of data from two or more stations. The bridging hub also does not provide any control of data flow. Furthermore, while the wiring scheme perts flexibility of system arrangements, there is no centralized maintenance.
It is apparent from the foregoing that each of the above described network topologies has its own advantages and disadvantages, and that the characteristics concerning privacy, security and reliability are different for each network topology. Each topology has its own requirements concerning network administration and maintenance. In many cases however the aforedescribed networks require active administration in adding and or deleting users and in many instances uniform software is required for all the connected data processing devices.
It is desirable to have a data distribution system that has the central control and maintenance of a star topology network and the flexibility and distribution wiring of a bus network, which is inexpensive and that can accommodate changing needs and varied distribution schemes of an actively used and changing local data distribution network in which data processing devices and/or stations are frequently added or deleted. It is also desirable that the distribution scheme be sufficiently flexible to allow additions to and removal of various components of the system without the necessity of active maintenance of an administrator. Any such distribution scheme must also take into account the desirability of hardware security, data privacy and ease of administration. A further consideration is whether a building must be rewired to install a local data distribution network as opposed to using existing wiring in the building.